Band gap engineering of amorphous A1-Ga-N alloys

ABSTRACT

A semiconductor structure and a scheme for forming a layer of amorphous material on a semiconductor substrate are provided. In accordance with one embodiment of the present invention, a semiconductor structure is provided comprising an amorphous alloy formed over at least a portion of a semiconductor substrate. The amorphous alloy comprises amorphous aluminum nitride (AlN) and amorphous gallium nitride (GaN). The amorphous alloy may be characterized by the following formula:  
     Al x Ga 1-x N  
     where x is a value greater than zero and less than one. The amorphous alloy may further comprise indium nitride. Relative proportions of aluminum and gallium in the amorphous aluminum gallium nitride alloy are controlled to engineer the band gap of the amorphous alloy.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation in part of U.S. patentapplication Ser. No. 09/431,339, filed Oct. 29, 1999.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This invention was made with government support under ContractNo. N00014-96-10782 awarded by Ballistic Missile Defense Organization.The Government has certain rights in this invention.

BACKGROUND OF THE INVENTION

[0003] The present invention relates to an improved semiconductormaterial and a method of its production. More specifically the presentinvention relates to an amorphous semiconductor alloy including aluminumand gallium and to a method of production that allows for convenientband gap engineering of the alloy and for deposition of the alloy on avariety of vacuum compatible materials.

[0004] Crystalline GaN, AlGaAs, GaAs, and InGaP have enjoyed success ina number of electronic and optical applications. For example, lightemitting diodes formed of AlGaAs, GaAs, and InGaP have been proposedusing epitaxial crystal growth techniques, see U.S. Pat. No. 4,971,928.UV detectors formed of crystalline Al_(x)Ga_(1-x)N are proposed in U.S.Pat. No. 4,616,248. The present invention is partially based upon therecognition that the success of these crystalline semiconductormaterials is limited by the various processes for their productionbecause these processes necessarily incorporate specific steps topreserve the crystalline state of the semiconductor material.

[0005] For example a typical semiconductor deposition scheme ispresented in U.S. Pat. No. 3,979,271, where solid layer semiconductorcompositions are deposited by simultaneously sputtering and dischargereacting materials and depositing the materials on a heated substrate.The substrate is typically heated above 300° C. to providepolycrystalline growth and typically to above 500° C. to provide highlyoriented, epitaxial growth. Higher temperatures may be needed forepitaxial growth on silicon substrates. The present inventors haverecognized that these particular heating steps limit the availability ofeconomical electronic and optical devices including conventionalcrystalline materials. Accordingly, there is a need for an improvedsemiconductor material and a more versatile method of depositing such asemiconductor material.

BRIEF SUMMARY OF THE INVENTION

[0006] This need is met by the present invention wherein a semiconductorstructure and a scheme for engineering a band gap of an amorphousmaterial and forming a layer of the amorphous material on asemiconductor substrate are provided. In accordance with one embodimentof the present invention, a semiconductor structure is providedcomprising an amorphous alloy formed over at least a portion of asemiconductor substrate. The amorphous alloy comprises aluminum nitride(AlN) and gallium nitride (GaN) and may be characterized by a band gapbetween about 3 eV and about 6 eV. The amorphous alloy may becharacterized by the following formula:

Al_(x)Ga_(1-x)N

[0007] where x is a value greater than zero and less than one.

[0008] The amorphous alloy may further comprise indium nitride and maybe characterized by a band gap between about 2 eV and about 6 eV. Adopant may be incorporated into the amorphous alloy. The dopant maycomprise a rare earth element or, more specifically, a rare earthluminescent center.

[0009] In accordance with another embodiment of the present invention, amethod of forming a layer of amorphous material on a semiconductorsubstrate is provided. The method comprising the steps of: (i)positioning the semiconductor substrate in a reactive sputter depositionchamber of a reactive sputtering system including at least one sputtertarget containing aluminum and gallium; (ii) introducing a nitrogen gasinto the sputter deposition chamber; (iii) operating the sputteringsystem to promote reaction of aluminum and gallium from the sputtertarget and nitrogen from the gas in the sputter deposition chamber; (iv)maintaining the semiconductor substrate at a deposition temperatureselected to promote growth of an amorphous aluminum gallium nitridealloy on the semiconductor substrate; and (v) further operating thesputtering system so as to designate relative proportions of aluminumand gallium in the amorphous aluminum gallium nitride alloy.

[0010] The sputter target may comprise a single integrated commontarget, a pair of discrete target portions, where one of the targetportions contains aluminum and the other of the target portions containsgallium, or a pair of targets, where one of the targets containsaluminum and the other of the targets contains gallium. The method mayfurther comprise the step of varying the relative proportions ofaluminum and gallium in the amorphous aluminum gallium nitride alloy soas to selectively control a band gap of the alloy.

[0011] The sputter target may further include indium and the method mayfurther comprise the steps of: (i) operating the sputtering system topromote reaction of aluminum, gallium, and indium from the sputtertarget and nitrogen from the gas in the sputter deposition chamber; (ii)maintaining the semiconductor substrate at a deposition temperatureselected to promote growth of an amorphous indium aluminum galliumnitride alloy on the semiconductor substrate; and (iii) operating thesputtering system so as to designate relative proportions of indium,aluminum, and gallium in the amorphous aluminum gallium nitride alloy.

[0012] The method may further comprise the step of introducing a dopantinto the amorphous alloy. The dopant may comprise a rare earth elementor, more specifically, a rare earth luminescent center.

[0013] In accordance with yet another embodiment of the presentinvention, a semiconductor material is provided comprising an amorphousalloy including amorphous aluminum nitride (AlN) and amorphous galliumnitride (GaN). The amorphous alloy may further include amorphous indiumnitride.

[0014] Accordingly, it is an object of the present invention to providean improved semiconductor structure and a convenient scheme for forminga layer of amorphous material on a semiconductor substrate. Otherobjects of the present invention will be apparent in light of thedescription of the invention embodied herein.

BRIEF DESCRIPTION OF THE DRAWING

[0015] The following detailed description of the preferred embodimentsof the present invention can be best understood when read in conjunctionwith FIG. 1, which is a schematic illustration of a system and methodfor forming a layer of amorphous material on a semiconductor substrateaccording to one aspect of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] A semiconductor structure 10 according to the present inventionis illustrated in FIG. 1 and comprises an amorphous alloy 20 formed overat least a portion of a semiconductor substrate 30.

[0017] The amorphous alloy 20 comprises amorphous aluminum nitride (AlN)and amorphous gallium nitride (GaN) and is characterized by a band gapbetween about 3 eV and about 6 eV. For the purposes of describing anddefining the present invention, it is noted that a layer of materialformed over a substrate may be partially embedded in the substrate, maybe formed directly on the substrate, or may be formed on an intermediatelayer or layers that are formed directly on the substrate.

[0018] The amorphous alloy 20 of the present invention is a wide bandgap semiconductor that may be doped and readily integrated with silicontechnology. Specifically, the amorphous alloy of the present inventionmay be easily deposited or grown on silicon and may be processed attemperatures that are compatible with the entire range of siliconfabrication technology. The amorphous alloy of the present invention mayalso be etched more easily than most crystalline nitrides.

[0019] In a specific embodiment of the present invention, the alloy isdoped with a rare earth luminescent center. The resulting rare earthluminescent device may be utilized for photonics application and may beengineered with band gaps optimized for efficient electroluminescence.Possible applications include wavelength dispersive multiplexingdevices, where sharp atomic resonance emission lines are particularlywell-suited for fiber-optic communication.

[0020] The amorphous alloy 20 of the present invention may be grown onthe semiconductor substrate 30 in a reactive sputtering system 40,illustrated schematically in FIG. 1. Specifically, the semiconductorsubstrate 30 is positioned on an anode 49 within a reactive sputterdeposition chamber 42 containing a carrier gas comprising nitrogen or amixture of nitrogen and argon or another inert gas and at least onesputter target 44. The sputter target 44 contains aluminum and galliumand may comprise a single integrated target containing both aluminum andgallium or a pair of discrete targets or target portions, one containingaluminum and the other containing gallium. For example, according to oneaspect of the present invention, the target 44 comprises a solidaluminum target where a portion of the aluminum is removed and replacedwith a gallium-filled reservoir. The aluminum portion of target 44 istypically made from aluminum having 99.999% purity, and the galliumportion of target 44 is typically made from gallium having 99.999%purity. The remaining components of the reactive sputtering system areconventional and include an RF source 45, a matching network 46, atarget cathode 48, and the anode 49.

[0021] Nitrogen gas is introduced into the sputter deposition chamber 42and the chamber is operated to encourage the necessary reactivesputtering of the aluminum, gallium, and nitrogen. Typically, argon oranother inert gas is also present in the sputter deposition chamber 42.The nitrogen gas introduced into the sputter deposition chambertypically is 99.9995% pure. The general manner in which the reactivesputtering of the present invention is initiated is not the subject ofthe present invention and may be readily gleaned from existing RFreactive sputtering technology. It is noted, however, that theparticular materials utilized in the reactive sputtering scheme of thepresent invention and the manner in which the sputter reaction iscontrolled to engineer the growth of the amorphous alloy 20 are thesubject matter of the present invention. Specifically, the respectivecompositions of the first and second sputter targets are selected tointroduce aluminum and gallium into the reaction process of the presentinvention. Further, the relative proportions of the aluminum and galliumintroduced into the reaction are controlled to engineer the band gap ofthe amorphous alloy 20 within, for example, a range of about 3 eV toabout 6 eV. Finally, the temperature of the substrate on which the alloyis grown is controlled at a relatively low value, e.g., between about77K to about 300K, to assure that the alloy grown on the substrate is anamorphous alloy 20.

[0022] The amorphous alloy 20 is characterized by the following formula:

Al_(x)Ga_(1-x)N

[0023] where the value of x is greater than zero and less than one. Theband gap of the amorphous aluminum gallium nitride alloy 20 is a directfunction of the relative proportions of aluminum and gallium in theamorphous alloy and these proportions are designated by suitable controlof the reactive sputtering system 40. For example, desired proportionsof aluminum and gallium in the amorphous alloy 20 may be generated bycontrolling or designating the relative proportions of aluminum andgallium in the sputter target 44. More specifically, a specific portionor portions of the surface area of the target 44 may be dedicated toaluminum and a corresponding portion or portions may be dedicated togallium. Alternatively, where two discrete targets or target portionsare utilized according to the present invention, the RF power to whicheach target or target portion is subject may be varied to control thedesired proportions of aluminum and gallium in the amorphous alloy 20.As can be seen from this description, the amorphous alloy 20 is formedon top of the substrate 30, but the formation process does not utilizethe components of the substrate 30 to form the amorphous alloy 20. As aresult, the amorphous alloy 20 does not contain impurities from thesubstrate. According to one embodiment of the present invention, theband gap lies between about 3 eV and about 6 eV. According to a furtherembodiment of the present invention, the sputter target 44 furtherincludes amorphous indium and the method further comprises the step ofoperating the sputtering system 40 to promote reaction of aluminum,gallium, nitrogen, and indium in the sputter deposition chamber 44 andgrowth of amorphous indium aluminum gallium nitride alloy on thesemiconductor substrate 30. Typically, the indium will have a purity of99.999%. As would be appreciated by those practicing the presentinvention, the sputtering system may be operated to designate relativeproportions of indium, aluminum, and gallium in the amorphous aluminumgallium nitride alloy. The band gap of the amorphous indium aluminumgallium nitride alloy may extend as low as about 2 eV.

[0024] The amorphous alloy 20 of the present invention does not containsubstantial amounts of hydrogen. The inclusion of hydrogen in anamorphous semiconductor film can cause passivation or compensation in adoped film. Hydrogen can occupy a site in the semiconductor in place ofthe intended dopant, and hydrogen can move through the semiconductor.Therefore, the inclusion of hydrogen in the semiconductor can passivatethe semiconductor and prevent the semiconductor from functioningproperly when doped. Additionally, the amorphous alloy 20 of the presentinvention does not contain detectable amounts of carbon. The inclusionof carbon in an amorphous semiconductor film can cause defect levels inthe films. Defect levels can cause the amorphous semiconductor tofunction poorly as a photoelectric material. Finally, the inclusion ofeven small amounts oxygen in the amorphous alloy 20 can raise the bandgap, thus affecting the linear variation in the band gap. Therefore, thetarget 44 of the present invention is generally 99.999% pure withrespect to hydrogen, carbon, and oxygen, in order to ensure that theamorphous alloy 20 does not contain substantial amounts of hydrogen,carbon, or oxygen.

[0025] The amorphous alloy of the present invention may containimpurities that do not affect the linear variation in band gap orelectronic properties. For example, the amorphous alloy could containimpurities such as copper or tin because these metals do not react withnitrogen. However, the target 44 of the present invention is generallyat least 99% pure with respect to metals in order to ensure a linearvariation in band gap. More typically, the target 44 will be 99.99% purewith respect to metals in order to ensure good electronic properties ofamorphous alloy 20.

[0026] For the purposes of describing and defining the presentinvention, it will be understood that the terms “consisting essentiallyof” and “substantially free of” with respect to the target 44 and theamorphous alloy 20 corresponds to the purity levels discussed above.

[0027] Having described the invention in detail and by reference topreferred embodiments thereof, it will be apparent that modificationsand variations are possible without departing from the scope of theinvention defined in the appended claims.

What is claimed is:
 1. A semiconductor structure comprising an amorphousalloy formed over at least a portion of a substrate, wherein saidamorphous alloy consists essentially of amorphous aluminum nitride (AlN)and amorphous gallium nitride (GaN) and is characterized by a band gapbetween about 2 eV and about 6 eV.
 2. A semiconductor structure asclaimed in claim 1 wherein said amorphous alloy is characterized by aband gap between about 3 eV and about 6 eV.
 3. A semiconductor structureas claimed in claim 1 wherein said amorphous alloy is characterized bythe following formula: Al_(x)Ga_(1-x)N where x is a value greater thanzero and less than one.
 4. A semiconductor structure comprising anamorphous alloy formed over at least a portion of a substrate, whereinsaid amorphous alloy consists essentially of amorphous aluminum nitride(AlN), amorphous gallium nitride (GaN), and a dopant and ischaracterized by a band gap between about 2 eV and about 6 eV.
 5. Asemiconductor structure as claimed in claim 4 wherein said dopantcomprises a rare earth element.
 6. A semiconductor structure as claimedin claim 5 wherein said dopant comprises a rare earth luminescentcenter.
 7. A semiconductor structure comprising an amorphous alloyformed over at least a portion of a substrate, wherein said amorphousalloy consists essentially of amorphous aluminum nitride (AlN),amorphous gallium nitride (GaN), and amorphous indium nitride (InN) andis characterized by a band gap between about 2 eV and about 6 eV.
 8. Asemiconductor structure as claimed in claim 7 wherein said amorphousalloy is characterized by a band gap between about 3 eV and about 6 eV.9. A semiconductor structure comprising an amorphous alloy formed overat least a portion of a substrate, wherein said amorphous alloy consistsessentially of amorphous aluminum nitride (AlN), amorphous galliumnitride (GaN), amorphous indium nitride (InN), and a dopant and ischaracterized by a band gap between about 2 eV and about 6 eV.
 10. Asemiconductor structure as claimed in claim 9 wherein said dopantcomprises a rare earth element.
 11. A semiconductor structure as claimedin claim 10 wherein said dopant comprises a rare earth luminescentcenter.
 12. A semiconductor structure comprising an amorphous alloyformed over at least a portion of a substrate, wherein said amorphousalloy comprises amorphous aluminum nitride (AlN) and amorphous galliumnitride (GaN), is substantially free of carbon, hydrogen, and oxygen,and is characterized by a band gap between about 2 eV and about 6 eV.13. A semiconductor structure as claimed in claim 12 wherein saidamorphous alloy is characterized by a band gap between about 3 eV andabout 6 eV.
 14. A semiconductor structure as claimed in claim 12 whereinsaid amorphous alloy is characterized by the following formula:Al_(x)Ga_(1-x)N where x is a value greater than zero and less than one.15. A semiconductor structure as claimed in claim 12 wherein saidamorphous alloy further comprises amorphous indium nitride.
 16. Asemiconductor structure as claimed in claim 12 further comprising adopant incorporated into said amorphous alloy.
 17. A semiconductorstructure as claimed in claim 16 wherein said dopant comprises a rareearth element.
 18. A semiconductor structure as claimed in claim 17wherein said dopant comprises a rare earth luminescent center.